Considering that there was a mistake on Delta's reps parts by giving you an inaccurate expiration date - I really do think that they should have done something.

However, to be fair, 730 dollars can't that big a sum of money if you managed to forget it, can it? I understand you're outraged due to the sum of money involved, but would you be as angry if it were a small sum that you were willing to write off? Mistakes were made on both sides, and it might be time to just take your lumps and move on.